Calling her "stunning, silly, smart, talented and funny," Kim Kardashian, 31, blogged that Jenner is "growing up way too fast for my liking, but what an amazing and beautiful young lady [she has] turned into. We are all so proud of [her] and I can't wait to see what the future has in store."

Kourtney Kardashian -- see exclusive photos of her newborn daughter, Penelope, in the new issue of Us Weekly, out now! -- also praised Jenner on her official blog. "You are filled with so much fun and happiness but so nurturing and understanding at the same time. You have wisdom beyond your years and I'm proud to call you my sister," the 33-year-old reality star wrote. "I love you madly. I hope all of your birthday wishes come true."